The adaptive line enhancer has significant applications in the enhancement of narrowband spectral lines in a broadband noise field when there is a poor signal-to-noise ratio at the input where there is insufficient a priori information on which to design appropriate filters. The device automatically filters out the components of the signal which are uncorrelated in time and passes the correlated portions. Since the properties of the device are determined solely by the input signal statistics, the properties of the filter automatically adjust to variations in the input signal statistics to obtain the least mean square (LMS) approximation to a Wiener-Hopf filter. The device will thus track slowly varying spectral lines in broadband noise. Specifically, properties are derived of an N-weight adaptive line enhancer for single stable input spectral line in both an arbitrary and a white noise background.
In a discussion of the theory of the invention, an analytic solution is obtained for the LMS weight vector in the signal-to-noise enhancement of an N-weight adaptive line enhancer for a single stable input spectral line in both an arbitrary broadband noise and white noise background. It is shown that the enhancer forms a narrowband filter in which the passband is centered on the frequency of the input spectral line. Also, for stable lines, the signal-to-noise enhancement depends only on the number of weights and is independent of the input signal-to-noise ratio. The enhancer adjusts the phase of the time-delayed input signals so that the components of the two channels, used in the filter, are in phase. The enhancer sets the amplitude of the filter weights so that the total signal and noise power within the passband is equal in the two parallel channels.
Background information useful for understanding this invention appears in the article by Widrow, B. et al, entitled "Adaptive Noise Cancelling: Principles and Applications", which appeared in the December 1975 issue of Proc. of the IEEE, Vol. 63, No. 12, Pp. 1692-1719.